1. Field of the Invention
This invention relates to plastic electrically insulting substrates for wiring circuit boards. More particularly, the present invention relates to plastic electrically insulating substrates for wiring circuit boards which have excellent thermal conductivity along the thickness direction.
2. Description of the Background
Wiring circuit boards are referred to as electrical parts on which electrical devices such as a transistor or IC, i.e. Integrated Circuit, is mounted and connected. Conventional wiring circuit boards are formed with electrically insulating substrates and conducting materials. Both inorganic and organic materials have been used as electrically insulating substrates. The properties required for insulating substrates include high electrically insulating resistability, dimensional stability, heat resistability, and high thermal conductivity for heat dissipation.
Polymers which have been used as insulating substrates include bismaleimide triazine resin, polyimide, epoxy, polybutadiene, and polytrifluoroethylene.
These polymers have reasonably good mechanical and electrical properties for wiring circuit boards However, none of these polymers have sufficient thermal conductivity, as compared to a ceramic insulating material, such as alumina (Al.sub.2 O.sub.3).
However, plastic substrates having high thermal conductivity are needed to dissipate the heat produced on front surface mountings of the aforesaid electrical device by operation of the aforesaid electrical devices or by thin film resistors to the back surface, namely in the direction of the thickness of substrates, in order to allow for the production of systems which are small and reliable.
Although studies regarding polymers which are inherently thermally conductive are relatively scanty, it has been found that when a semicrystalline polymer is oriented by, for example, drawing, the thermal conductivity only taken along the orientation direction increases rapidly while the thermal conductivity in the perpendicular direction shows a moderate decrease. However these drawn semicrystalline polymers cannot be used as electrically insulating substrates for wiring circuit boards because the drawn semicrystalline polymer sheets do not conduct heat well along the thickness direction. For electrically insulating substrates, it is essential that heat produced by the aforesaid electrical devices mounted on the front surface of substrates be conducted to the back surface for heat dissipation in order to increase the effective area of heat dissipation.